1. Field of the Invention
The present invention relates to an optical device, a surface emitting type device and a method for manufacturing the same.
2. Related Background Art
In various kinds of optical devices having a light emitting function, a light detecting function, a light modulating function, and the like, a distributed Bragg reflector (herein after referred to as DBR) having a high reflectance is used. The DBR is a reflector in which two kinds of materials having different refractive indexes are alternately laminated to reflect light by the use of a difference in the refractive index. A surface emitting type laser will hereinafter be described as an example of an optical device using the DBR.
The surface emitting type laser is a surface emitting type device which emits laser beam in a direction perpendicular to a substrate. This device is easily integrated two-dimensionally and is expected to be applied to a parallel optical information processing, an optical interconnection, or a data storage field such as an optical disc. As the surface emitting type lasers have been developed a GaInAs/GaAs base surface emitting type laser having a wavelength of 0.98 μm, a GaAlAs/GaAs base surface emitting type laser having a wavelength of 0.78 μm to 0.85 μm, a AlGaInP/GaAs base surface emitting type laser having a wavelength of 0.63 μm to 0.67 μm. These surface emitting type lasers generally include a cavity comprising a n-type clad layer, an active layer and a p-type clad layer. And the lasers generally include DBRs formed on the upper and lower sides of the cavity. In the surface emitting type laser, it is necessary to make the reflectance of the DBRs, disposed at the upper and lower sides of the active layer, 99% or more.
In recent years, attention has been paid to a nitride semiconductor base laser using a nitride semiconductor having a large band gap and having a short wavelength of about 0.4 μm. Such a short wavelength laser has various advantages such as increasing the packing density of an optical disc such as a DVD. For this reason, this nitride semiconductor base laser receives attention as the light source of the next generation of a high-density optical disc system, or the like.
However, a surface emitting type laser has not been put into practical use in the nitride semiconductor base laser. For one thing, this is because it has been difficult to make a DBR having a high reflectance described above.
That is, in the case of a nitride gallium base laser, a combination of semiconductor materials usable for the DBR includes a combination of GaN and AlGaN and a combination of GaN and AlN. However, even in the case where the DBR is constituted by a combination of GaN (n=2.57) and AlN (n=2.15), in which a difference in refractive index n is large, it is necessary to grow a multilayer film of 20 layers or more, in a minimum, so as to produce a required high reflectance. Accordingly, in the surface emitting type laser, it is necessary to grow a multilayer film of 40 layers or more in total of the upper and lower DBRs. However, in the case of a laminated structure of GaN and AlN, there is a large difference in lattice constant and an Al crystal is hard, so cracks are apt to be produced. For this reason, when the multilayer film of 40 layers or more is formed, the occurrence of the cracks can not be avoided, and manufacturing yield remarkably decreases. Further, since the growth rate of the multilayer film of GaN and AlN is slow, the forming of the multilayer film of 40 layers or more presents a problem of remarkably reducing productivity. In this manner, if a reflector having a high reflectance is formed of the nitride semiconductor, the reflector becomes a laminated structure of many layers, which reduces the manufacturing yield and productivity. Further, in the case of this DBR, a high reflection band (wavelength width of stop band) is very narrow. So, in the case of this DBR, oscillation conditions can not be satisfied even if the thickness of each layer of the DBR, the thickness of the cavity or the composition of the active layer are shifted a little from the design values. Hence, in the case of this DBR, stable laser beam can not be produced. From these reasons, the surface emitting type nitride semiconductor laser has not been put into practical use.